Deep sleep, also known as Stage 3 NREM or "slow-wave sleep," is the most physically restorative phase of your nightly cycle. During this time, your brain waves slow down significantly into delta waves, and your body focuses entirely on repair: it heals tissues, builds muscle, and strengthens the immune system. Because your heart rate and breathing are at their lowest points, it is the hardest stage to wake up from, and doing so often leaves you feeling "groggy." From a spiritual perspective, many believe this is the time when the conscious mind completely steps aside, allowing for the deepest energetic recharging and soul-level rest.